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* Add i256 (#2637) * RAT * Fix doc comments * Store as parts * Custom multiply implementation * Make from_parts public * Document mulx * Remove branch from to_i128 * Clippy
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// Licensed to the Apache Software Foundation (ASF) under one | ||
// or more contributor license agreements. See the NOTICE file | ||
// distributed with this work for additional information | ||
// regarding copyright ownership. The ASF licenses this file | ||
// to you under the Apache License, Version 2.0 (the | ||
// "License"); you may not use this file except in compliance | ||
// with the License. You may obtain a copy of the License at | ||
// | ||
// http://www.apache.org/licenses/LICENSE-2.0 | ||
// | ||
// Unless required by applicable law or agreed to in writing, | ||
// software distributed under the License is distributed on an | ||
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY | ||
// KIND, either express or implied. See the License for the | ||
// specific language governing permissions and limitations | ||
// under the License. | ||
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use num::BigInt; | ||
use std::cmp::Ordering; | ||
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/// A signed 256-bit integer | ||
#[allow(non_camel_case_types)] | ||
#[derive(Copy, Clone, Default, Eq, PartialEq, Hash)] | ||
pub struct i256 { | ||
low: u128, | ||
high: i128, | ||
} | ||
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impl std::fmt::Debug for i256 { | ||
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { | ||
write!(f, "{}", self) | ||
} | ||
} | ||
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impl std::fmt::Display for i256 { | ||
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result { | ||
write!(f, "{}", BigInt::from_signed_bytes_le(&self.to_le_bytes())) | ||
} | ||
} | ||
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impl PartialOrd for i256 { | ||
fn partial_cmp(&self, other: &Self) -> Option<Ordering> { | ||
Some(self.cmp(other)) | ||
} | ||
} | ||
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impl Ord for i256 { | ||
fn cmp(&self, other: &Self) -> Ordering { | ||
// This is 25x faster than using a variable length encoding such | ||
// as BigInt as it avoids allocation and branching | ||
self.high.cmp(&other.high).then(self.low.cmp(&other.low)) | ||
} | ||
} | ||
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impl i256 { | ||
/// The additive identity for this integer type, i.e. `0`. | ||
pub const ZERO: Self = i256 { low: 0, high: 0 }; | ||
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/// The multiplicative identity for this integer type, i.e. `1`. | ||
pub const ONE: Self = i256 { low: 1, high: 0 }; | ||
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/// The multiplicative inverse for this integer type, i.e. `-1`. | ||
pub const MINUS_ONE: Self = i256 { | ||
low: u128::MAX, | ||
high: -1, | ||
}; | ||
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/// Create an integer value from its representation as a byte array in little-endian. | ||
#[inline] | ||
pub fn from_le_bytes(b: [u8; 32]) -> Self { | ||
Self { | ||
high: i128::from_le_bytes(b[16..32].try_into().unwrap()), | ||
low: u128::from_le_bytes(b[0..16].try_into().unwrap()), | ||
} | ||
} | ||
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/// Create an i256 from the provided low u128 and high i128 | ||
#[inline] | ||
pub fn from_parts(low: u128, high: i128) -> Self { | ||
Self { low, high } | ||
} | ||
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/// Returns this `i256` as a low u128 and high i128 | ||
pub fn to_parts(self) -> (u128, i128) { | ||
(self.low, self.high) | ||
} | ||
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/// Converts this `i256` into an `i128` returning `None` if this would result | ||
/// in truncation/overflow | ||
pub fn to_i128(self) -> Option<i128> { | ||
let as_i128 = self.low as i128; | ||
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let high_negative = self.high < 0; | ||
let low_negative = as_i128 < 0; | ||
let high_valid = self.high == -1 || self.high == 0; | ||
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(high_negative == low_negative && high_valid).then_some(self.low as i128) | ||
} | ||
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/// Return the memory representation of this integer as a byte array in little-endian byte order. | ||
#[inline] | ||
pub fn to_le_bytes(self) -> [u8; 32] { | ||
let mut t = [0; 32]; | ||
let t_low: &mut [u8; 16] = (&mut t[0..16]).try_into().unwrap(); | ||
*t_low = self.low.to_le_bytes(); | ||
let t_high: &mut [u8; 16] = (&mut t[16..32]).try_into().unwrap(); | ||
*t_high = self.high.to_le_bytes(); | ||
t | ||
} | ||
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/// Create an i256 from the provided [`BigInt`] returning a bool indicating | ||
/// if overflow occurred | ||
fn from_bigint_with_overflow(v: BigInt) -> (Self, bool) { | ||
let v_bytes = v.to_signed_bytes_le(); | ||
match v_bytes.len().cmp(&32) { | ||
Ordering::Less => { | ||
let mut bytes = if num::Signed::is_negative(&v) { | ||
[255_u8; 32] | ||
} else { | ||
[0; 32] | ||
}; | ||
bytes[0..v_bytes.len()].copy_from_slice(&v_bytes[..v_bytes.len()]); | ||
(Self::from_le_bytes(bytes), false) | ||
} | ||
Ordering::Equal => (Self::from_le_bytes(v_bytes.try_into().unwrap()), false), | ||
Ordering::Greater => { | ||
(Self::from_le_bytes(v_bytes[..32].try_into().unwrap()), true) | ||
} | ||
} | ||
} | ||
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/// Performs wrapping addition | ||
#[inline] | ||
pub fn wrapping_add(self, other: Self) -> Self { | ||
let (low, carry) = self.low.overflowing_add(other.low); | ||
let high = self.high.wrapping_add(other.high).wrapping_add(carry as _); | ||
Self { low, high } | ||
} | ||
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/// Performs checked addition | ||
#[inline] | ||
pub fn checked_add(self, other: Self) -> Option<Self> { | ||
let (low, carry) = self.low.overflowing_add(other.low); | ||
let high = self.high.checked_add(other.high)?.checked_add(carry as _)?; | ||
Some(Self { low, high }) | ||
} | ||
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/// Performs wrapping subtraction | ||
#[inline] | ||
pub fn wrapping_sub(self, other: Self) -> Self { | ||
let (low, carry) = self.low.overflowing_sub(other.low); | ||
let high = self.high.wrapping_sub(other.high).wrapping_sub(carry as _); | ||
Self { low, high } | ||
} | ||
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/// Performs checked subtraction | ||
#[inline] | ||
pub fn checked_sub(self, other: Self) -> Option<Self> { | ||
let (low, carry) = self.low.overflowing_sub(other.low); | ||
let high = self.high.checked_sub(other.high)?.checked_sub(carry as _)?; | ||
Some(Self { low, high }) | ||
} | ||
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/// Performs wrapping multiplication | ||
#[inline] | ||
pub fn wrapping_mul(self, other: Self) -> Self { | ||
let (low, high) = mulx(self.low, other.low); | ||
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// Compute the high multiples, only impacting the high 128-bits | ||
let hl = self.high.wrapping_mul(other.low as i128); | ||
let lh = (self.low as i128).wrapping_mul(other.high); | ||
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Self { | ||
low, | ||
high: (high as i128).wrapping_add(hl).wrapping_add(lh), | ||
} | ||
} | ||
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/// Performs checked multiplication | ||
#[inline] | ||
pub fn checked_mul(self, other: Self) -> Option<Self> { | ||
let (low, high) = mulx(self.low, other.low); | ||
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// Compute the high multiples, only impacting the high 128-bits | ||
let hl = self.high.checked_mul(other.low as i128)?; | ||
let lh = (self.low as i128).checked_mul(other.high)?; | ||
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Some(Self { | ||
low, | ||
high: (high as i128).checked_add(hl)?.checked_add(lh)?, | ||
}) | ||
} | ||
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/// Performs wrapping division | ||
#[inline] | ||
pub fn wrapping_div(self, other: Self) -> Self { | ||
let l = BigInt::from_signed_bytes_le(&self.to_le_bytes()); | ||
let r = BigInt::from_signed_bytes_le(&other.to_le_bytes()); | ||
Self::from_bigint_with_overflow(l / r).0 | ||
} | ||
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/// Performs checked division | ||
#[inline] | ||
pub fn checked_div(self, other: Self) -> Option<Self> { | ||
let l = BigInt::from_signed_bytes_le(&self.to_le_bytes()); | ||
let r = BigInt::from_signed_bytes_le(&other.to_le_bytes()); | ||
let (val, overflow) = Self::from_bigint_with_overflow(l / r); | ||
(!overflow).then_some(val) | ||
} | ||
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/// Performs wrapping remainder | ||
#[inline] | ||
pub fn wrapping_rem(self, other: Self) -> Self { | ||
let l = BigInt::from_signed_bytes_le(&self.to_le_bytes()); | ||
let r = BigInt::from_signed_bytes_le(&other.to_le_bytes()); | ||
Self::from_bigint_with_overflow(l % r).0 | ||
} | ||
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/// Performs checked remainder | ||
#[inline] | ||
pub fn checked_rem(self, other: Self) -> Option<Self> { | ||
if other == Self::ZERO { | ||
return None; | ||
} | ||
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let l = BigInt::from_signed_bytes_le(&self.to_le_bytes()); | ||
let r = BigInt::from_signed_bytes_le(&other.to_le_bytes()); | ||
let (val, overflow) = Self::from_bigint_with_overflow(l % r); | ||
(!overflow).then_some(val) | ||
} | ||
} | ||
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/// Performs an unsigned multiplication of `a * b` returning a tuple of | ||
/// `(low, high)` where `low` contains the lower 128-bits of the result | ||
/// and `high` the higher 128-bits | ||
/// | ||
/// This mirrors the x86 mulx instruction but for 128-bit types | ||
#[inline] | ||
fn mulx(a: u128, b: u128) -> (u128, u128) { | ||
let split = |a: u128| (a & (u64::MAX as u128), a >> 64); | ||
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const MASK: u128 = u64::MAX as _; | ||
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let (a_low, a_high) = split(a); | ||
let (b_low, b_high) = split(b); | ||
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// Carry stores the upper 64-bits of low and lower 64-bits of high | ||
let (mut low, mut carry) = split(a_low * b_low); | ||
carry += a_high * b_low; | ||
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// Update low and high with corresponding parts of carry | ||
low += carry << 64; | ||
let mut high = carry >> 64; | ||
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// Update carry with overflow from low | ||
carry = low >> 64; | ||
low &= MASK; | ||
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// Perform multiply including overflow from low | ||
carry += b_high * a_low; | ||
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// Update low and high with values from carry | ||
low += carry << 64; | ||
high += carry >> 64; | ||
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// Perform 4th multiplication | ||
high += a_high * b_high; | ||
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(low, high) | ||
} | ||
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#[cfg(test)] | ||
mod tests { | ||
use super::*; | ||
use num::{BigInt, FromPrimitive, ToPrimitive}; | ||
use rand::{thread_rng, Rng}; | ||
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#[test] | ||
fn test_signed_cmp() { | ||
let a = i256::from_parts(i128::MAX as u128, 12); | ||
let b = i256::from_parts(i128::MIN as u128, 12); | ||
assert!(a < b); | ||
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let a = i256::from_parts(i128::MAX as u128, 12); | ||
let b = i256::from_parts(i128::MIN as u128, -12); | ||
assert!(a > b); | ||
} | ||
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#[test] | ||
fn test_to_i128() { | ||
let vals = [ | ||
BigInt::from_i128(-1).unwrap(), | ||
BigInt::from_i128(i128::MAX).unwrap(), | ||
BigInt::from_i128(i128::MIN).unwrap(), | ||
BigInt::from_u128(u128::MIN).unwrap(), | ||
BigInt::from_u128(u128::MAX).unwrap(), | ||
]; | ||
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for v in vals { | ||
let (t, overflow) = i256::from_bigint_with_overflow(v.clone()); | ||
assert!(!overflow); | ||
assert_eq!(t.to_i128(), v.to_i128(), "{} vs {}", v, t); | ||
} | ||
} | ||
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#[test] | ||
fn test_i256() { | ||
let mut rng = thread_rng(); | ||
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for _ in 0..1000 { | ||
let mut l = [0_u8; 32]; | ||
let len = rng.gen_range(0..32); | ||
l.iter_mut().take(len).for_each(|x| *x = rng.gen()); | ||
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let mut r = [0_u8; 32]; | ||
let len = rng.gen_range(0..32); | ||
r.iter_mut().take(len).for_each(|x| *x = rng.gen()); | ||
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let il = i256::from_le_bytes(l); | ||
let ir = i256::from_le_bytes(r); | ||
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let bl = BigInt::from_signed_bytes_le(&l); | ||
let br = BigInt::from_signed_bytes_le(&r); | ||
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// Comparison | ||
assert_eq!(il.cmp(&ir), bl.cmp(&br), "{} cmp {}", bl, br); | ||
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// To i128 | ||
assert_eq!(il.to_i128(), bl.to_i128(), "{}", bl); | ||
assert_eq!(ir.to_i128(), br.to_i128(), "{}", br); | ||
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// Addition | ||
let actual = il.wrapping_add(ir); | ||
let (expected, overflow) = | ||
i256::from_bigint_with_overflow(bl.clone() + br.clone()); | ||
assert_eq!(actual, expected); | ||
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let checked = il.checked_add(ir); | ||
match overflow { | ||
true => assert!(checked.is_none()), | ||
false => assert_eq!(checked.unwrap(), actual), | ||
} | ||
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// Subtraction | ||
let actual = il.wrapping_sub(ir); | ||
let (expected, overflow) = | ||
i256::from_bigint_with_overflow(bl.clone() - br.clone()); | ||
assert_eq!(actual.to_string(), expected.to_string()); | ||
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let checked = il.checked_sub(ir); | ||
match overflow { | ||
true => assert!(checked.is_none()), | ||
false => assert_eq!(checked.unwrap(), actual), | ||
} | ||
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// Multiplication | ||
let actual = il.wrapping_mul(ir); | ||
let (expected, overflow) = | ||
i256::from_bigint_with_overflow(bl.clone() * br.clone()); | ||
assert_eq!(actual.to_string(), expected.to_string()); | ||
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let checked = il.checked_mul(ir); | ||
match overflow { | ||
true => assert!(checked.is_none()), | ||
false => assert_eq!(checked.unwrap(), actual), | ||
} | ||
} | ||
} | ||
} |
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